#pragma once

#include <unordered_map>
#include <list>
#include <mutex>
#include <memory>
#include <functional>

/*
 * 缓存淘汰策略 - 最近最少使用LRU(Least Recently Used);
 * 每次访问(读/写)一个元素, 就把它标记为最近使用。
 * 当缓存满了, 就淘汰最久没用的元素。
 */

/**
 * @brief Thread-safe LRU (Least Recently Used) Cache template
 *
 * This template provides a generic LRU cache implementation with the following features:
 * - Thread-safe operations using mutex
 * - Configurable capacity with automatic eviction
 * - O(1) get, put, and remove operations
 * - Optional eviction callback for cleanup
 *
 * @tparam Key The key type (must be hashable)
 * @tparam Value The value type
 */
template <typename Key, typename Value>
class LRUCache
{
public:
    using key_type = Key;
    using value_type = Value;
    using eviction_callback_t = std::function<void(const Key &, const Value &)>;

private:
    // Double-linked list node for LRU ordering
    struct Node
    {
        Key key;
        Value value;

        Node(const Key &k, const Value &v) :
            key(k), value(v)
        {
        }
    };

    using node_list_t = std::list<Node>;
    using node_iterator_t = typename node_list_t::iterator;
    using hash_map_t = std::unordered_map<Key, node_iterator_t>;

public:
    /**
     * @brief Construct LRUCache with specified capacity
     * @param capacity Maximum number of items to cache (default: 100)
     */
    explicit LRUCache(size_t capacity = 100) :
        capacity_(capacity), eviction_callback_(nullptr)
    {
        if (capacity_ == 0)
        {
            capacity_ = 1; // Minimum capacity of 1
        }
    }

    /**
     * @brief Destructor
     */
    virtual ~LRUCache()
    {
        clear();
    }

    // Disable copy constructor and assignment operator
    LRUCache(const LRUCache &) = delete;
    LRUCache &operator=(const LRUCache &) = delete;

    /**
     * @brief Set eviction callback function
     * @param callback Function to call when items are evicted
     */
    void set_eviction_callback(eviction_callback_t callback)
    {
        std::lock_guard<std::mutex> lock(mutex_);
        eviction_callback_ = callback;
    }

    /**
     * @brief Get value by key
     * @param key The key to search for
     * @param value Output parameter for the value
     * @return true if key exists, false otherwise
     */
    bool get(const Key &key, Value &value)
    {
        std::lock_guard<std::mutex> lock(mutex_);
        auto it = hash_map_.find(key);
        if (it == hash_map_.end())
        {
            return false;
        }

        // Move to front (most recently used)
        move_to_front(it->second);
        value = it->second->value;
        return true;
    }

    /**
     * @brief Get value by key (alternative interface)
     * @param key The key to search for
     * @return Pointer to value if exists, nullptr otherwise
     */
    Value *get(const Key &key)
    {
        std::lock_guard<std::mutex> lock(mutex_);
        auto it = hash_map_.find(key);
        if (it == hash_map_.end())
        {
            return nullptr;
        }

        // Move to front (most recently used)
        move_to_front(it->second);
        return &(it->second->value);
    }

    /**
     * @brief Put key-value pair into cache
     * @param key The key
     * @param value The value
     * @return true if new item was added, false if existing item was updated
     */
    bool put(const Key &key, const Value &value)
    {
        std::lock_guard<std::mutex> lock(mutex_);
        auto it = hash_map_.find(key);

        if (it != hash_map_.end())
        {
            // Update existing item
            it->second->value = value;
            move_to_front(it->second);
            return false;
        }

        // Add new item
        if (node_list_.size() >= capacity_)
        {
            evict_lru();
        }

        node_list_.emplace_front(key, value);
        hash_map_[key] = node_list_.begin();
        return true;
    }

    /**
     * @brief Remove item by key
     * @param key The key to remove
     * @return true if item was removed, false if key not found
     */
    bool remove(const Key &key)
    {
        std::lock_guard<std::mutex> lock(mutex_);
        auto it = hash_map_.find(key);
        if (it == hash_map_.end())
        {
            return false;
        }

        // Call eviction callback if set
        if (eviction_callback_)
        {
            eviction_callback_(it->second->key, it->second->value);
        }

        node_list_.erase(it->second);
        hash_map_.erase(it);
        return true;
    }

    /**
     * @brief Check if key exists in cache
     * @param key The key to check
     * @return true if key exists, false otherwise
     */
    bool contains(const Key &key) const
    {
        std::lock_guard<std::mutex> lock(mutex_);
        return hash_map_.find(key) != hash_map_.end();
    }

    /**
     * @brief Clear all items from cache
     */
    void clear()
    {
        std::lock_guard<std::mutex> lock(mutex_);
        if (eviction_callback_)
        {
            for (const auto &node : node_list_)
            {
                eviction_callback_(node.key, node.value);
            }
        }
        node_list_.clear();
        hash_map_.clear();
    }

    /**
     * @brief Get current cache size
     * @return Number of items in cache
     */
    size_t size() const
    {
        std::lock_guard<std::mutex> lock(mutex_);
        return node_list_.size();
    }

    /**
     * @brief Get cache capacity
     * @return Maximum number of items cache can hold
     */
    size_t capacity() const
    {
        return capacity_;
    }

    /**
     * @brief Check if cache is empty
     * @return true if cache is empty, false otherwise
     */
    bool empty() const
    {
        std::lock_guard<std::mutex> lock(mutex_);
        return node_list_.empty();
    }

    /**
     * @brief Set new capacity (may trigger eviction)
     * @param new_capacity New capacity value
     */
    void set_capacity(size_t new_capacity)
    {
        if (new_capacity == 0)
        {
            new_capacity = 1; // Minimum capacity of 1
        }

        std::lock_guard<std::mutex> lock(mutex_);
        capacity_ = new_capacity;

        // Evict excess items if necessary
        while (node_list_.size() > capacity_)
        {
            evict_lru();
        }
    }

    /**
     * @brief Apply a function to all cached items (for iteration)
     * @param func Function to apply to each key-value pair
     * Note: This is provided for compatibility but should be used carefully
     * as it may affect performance due to locking
     */
    template <typename Func>
    void for_each(Func func)
    {
        std::lock_guard<std::mutex> lock(mutex_);
        for (const auto &node : node_list_)
        {
            func(node.key, node.value);
        }
    }

    /**
     * @brief Remove items that match a predicate
     * @param predicate Function that returns true for items to remove
     * @return Number of items removed
     */
    template <typename Predicate>
    size_t remove_if(Predicate predicate)
    {
        std::lock_guard<std::mutex> lock(mutex_);
        size_t removed_count = 0;

        auto it = node_list_.begin();
        while (it != node_list_.end())
        {
            if (predicate(it->key, it->value))
            {
                // Call eviction callback if set
                if (eviction_callback_)
                {
                    eviction_callback_(it->key, it->value);
                }

                hash_map_.erase(it->key);
                it = node_list_.erase(it);
                removed_count++;
            }
            else
            {
                ++it;
            }
        }

        return removed_count;
    }

protected:
    /**
     * @brief Move node to front of list (most recently used position)
     * @param it Iterator to the node to move
     */
    void move_to_front(node_iterator_t it)
    {
        if (it != node_list_.begin())
        {
            node_list_.splice(node_list_.begin(), node_list_, it);
        }
    }

    /**
     * @brief Evict least recently used item
     */
    void evict_lru()
    {
        if (node_list_.empty())
        {
            return;
        }

        auto last = std::prev(node_list_.end());

        // Call eviction callback if set
        if (eviction_callback_)
        {
            eviction_callback_(last->key, last->value);
        }

        hash_map_.erase(last->key);
        node_list_.erase(last);
    }

protected:
    size_t capacity_;                       // Maximum cache capacity
    mutable std::mutex mutex_;              // Mutex for thread safety
    node_list_t node_list_;                 // Doubly-linked list for LRU ordering
    hash_map_t hash_map_;                   // Hash map for O(1) access
    eviction_callback_t eviction_callback_; // Optional eviction callback
};
